1. Field of the Invention
The invention relates to an operating method for a refuse processing furnace in which refuse like incineration residue or sewer sludge is melted.
2. Prior Art
When collected urban garbage is incinerated in a burning furnace, there remains incineration residue, or ash, which may contain harmful heavy metal elements as chromium, zinc, lead, cadmium, mercury, etc. Sewer sludge is another harmful refuse that may contain such heavy metal elements. Therefore, the natural dumping of such residue or refuse is generally regulated or prohibited in some countries.
In order to make the refuse safe, many measures have been taken. It is buried into deep underground. In this case, however, the heavy metal elements may infiltrate into the environment soil and dissolve into the underground water.
It is then proposed that the refuse is processed by melting with a reflection furnace, an induction furnace or an electric arc furnace. These melting processes have advantages that the melted and solidified material has less volume compared to the unprocessed refuse and the harmful heavy metals will not come out of the solidified material. Among these processes, the electric arc furnace process is shown, for example, in the Japanese Published Unexamined Patent Application No. Sho 52-86976. There are still three variations in the electric arc furnace melting process. One is called an open arc process in which naked electrodes are kept apart from uncovered surface of molten refuse during the process and the electric discharge arc is generated between the electrodes and the molten refuse. Another is called a resistance heating process, in which electrodes are plunged into unmolten refuse floating over molten refuse, electric current between those electrodes goes through the unmolten refuse and the refuse is melted by the heat generated by the Joule effect or the resistance heat. The last one is called a submerged arc heating process in which the two preceding methods are combined.
An example of the submerged arc heating process is shown in the Japanese Published Examined Patent Application No. Sho 57-55476. In this method, the electrodes of the furnace are plunged into a heap of unmolten refuse floating over the molten refuse in the furnace. The discharge arc is generated between the electrodes and the molten refuse, while the resistance heat is also generated within the unmolten refuse. This heating method has an advantage that the heat radiation from the molten surface is much reduced because it is prevented by the unmolten refuse covering the molten surface. Therefore, the input electric power can be reduced and apparatuses on the top of the furnace are protected from high temperature. Another advantage of the heating method is that emergence of dust from the molten surface caused by strikes of the electric discharge arc is also reduced.
The assignee of the present invention has proposed another example of such submerged arc heating process in the Japanese Published Unexamined Patent Application No. Sho 52-143965. In the method, refuse or sludge is put on molten base metal material, e.g., molten iron, in the furnace. The refuse or sludge is then melted by the abovementioned submerge arc process with reducing atmosphere. The heavy metals in the refuse or sludge is transferred to and dissolved in the base molten metal and also in the molten slag (melted refuse or sludge) on the molten base metal.
In these operating methods for refuse furnaces, there still remains problems as follows. The input amount of electric power and the charging amount of refuse are not well controlled. The electric power is controlled by changing voltage with a tap operation. Since there is a large variety in metal content of the refuse, the electric resistance of the refuse varies as well, requiring frequent manual change of the tap. The refuse charging is, at most, controlled depending on the detected temperature at the top of the furnace. When the ventilativeness or heat transfer property of the unmolten refuse varies, the detected temperature does not indicate the heaped amount of unmolten refuse on the molten refuse. In any case, the problems are that the discharge flow of molten refuse out of the furnace cannot be regulated to be constant and tedious manual operations are required during the process.